chore: Remove andrewsg from CODEOWNERS (#812)

Author: rwhogg

packages/google-cloud-ndb/.github/CODEOWNERS

@@ -2,4 +2,4 @@
 # This file controls who is tagged for review for any given pull request.
 
 # These are the default owners
-*                     @googleapis/api-firestore @googleapis/yoshi-python @andrewsg
+*                     @googleapis/api-firestore @googleapis/yoshi-python